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A turbo button refers to a button on a piece of electronic equipment, which makes the equipment run faster (or slower) in some way. Although the name is based on that of a forced induction air compressor which makes a car go faster, automotive turbochargers are not manually operated, but are directly linked to the engine's output. However, it is possible to use a pushbutton to actuate a wastegate, effectively bypassing the turbocharger.
Personal computers 
On personal computers, the turbo button changes the effective speed of the system. It usually accomplishes this by either adjusting the CPU clock speed directly, or by turning off the processor's cache, forcing it to rely on the significantly slower main memory for memory accesses. The button was generally present on older systems, and was designed to allow the user to play older games that depended on processor speed for their timing.
Some systems also supported keyboard combinations Ctrl-Alt-+ and Ctrl-Alt-- for switching turbo mode on and off; ITT Xtra used Ctrl-Alt-\ to toggle.
Calling it a "turbo" button when its function slows the system down can be a bit misleading, but the button was usually set up so the system would be at full speed when the button was "on". The turbo button was often linked to a MHz LED display on the system case, or to a "hi"/"lo" LED display.
While the implementation of the turbo button by manufacturers has all but disappeared, software developers have compensated with software replacements. One example is DOSBox, which offers full turbo button functionality with adjustable clock speed. Modern PCs that support ACPI power management may provide software controls to switch ACPI performance states or other CPU throttling modes.
Some keyboards have "Turbo" buttons that adjust the computer performance or the keyboard repeat rate.
Video games 
On some video game controllers, a Turbo button or Autofire (sometimes implemented as a sliding switch instead of a button) determines the repeat rate of another action button. For example, the Nintendo Entertainment System's controller has two action buttons, labeled "A" and "B". Normally, pressing the "A" button will result in the action associated with "A" being done once—for example, a character will jump once. This happens even when the "A" button is held down (depressed continually). An enhanced or upgraded controller's "Turbo" function will change this held-down functionality, so that the character would jump repeatedly, as if the "A" button were being pressed many times very quickly (a desirable feature in games where, for example, the "A" button fires a projectile or rapid pressing of a button is required).